Traffic on a switching system may be thought of as either TDM (Time-Division Multiplexed) traffic or PDU (Protocol Data Unit) traffic. TDM traffic has stricter requirements on latency, jitter, and bit errors as compared to PDU traffic. Because of the characteristic differences in the two types of traffic, switching is traditionally performed on different types of switching fabrics.
TDM traffic is typically switched over a circuit switching fabric, which provides a dedicated line for a TDM stream from an ingress port/device to an egress port/device. A so-called TDM system, or a switching system with a TDM fabric (based on circuit switching) will also typically use a timeslot allocation mechanism to compute circuit allocations through the fabric. The use of the concepts of timeslots and timeslot allocation in conjunction with the circuit switching provides for a switching mechanism that allows a dedicated path for a TDM stream for an allotted period of time. Some systems will also allow for rearrangement of the timeslot allocation. Such systems then provide a time-space (reallocation of timeslots-circuit switching) switch for switching TDM traffic. Circuit switching provides the benefit to TDM traffic that it is deterministic, and the use of a circuit by one TDM stream comes at the exclusion of all other streams. However, data traffic is typically not switched with a circuit-switching mechanism.
In contrast to circuit-switched systems are packet-switched systems. A packet-switched (or cell-switched) system deals with ingress/egress data in single packets, or cells. As used herein, “packet” and “cell” are to be understood to be interchangeable. They refer to a packet, a cell, or any other similar data structure. For example, they may refer to group of bits, where certain bits represent a payload, and other bits may represent a header, or some sort of addressing bits to enable routing of the cell/packet to its proper destination. Rather than having a dedicated circuit over which to transmit data, a packet-switched system uses a scheduler to arbitrate among cells contending for the same output port/device. PDU data is dealt with on a single packet/cell basis in the switching fabric. The scheduler may use a number of methods known in the art to determine the best allocation of cells among available output ports, at least including algorithms based on traffic priority, fair queuing, round robin, etc.
Efforts have been made to use a single switch to handle both TDM and PDU traffic. Current methods involve the use of hybrid fabrics as in FIG. 1, or TDM emulation as in FIG. 2. Referring to FIG. 1, a prior art system is illustrated with a native TDM (i.e., circuit switched) fabric. TDM linecards 111 are ingress/egress devices dedicated to TDM traffic. TDM traffic is received from a network (not shown), switched over TDM fabric 101 via circuit switching as discussed above, and returned to TDM linecards 111 to forward the TDM traffic to its destination on the network. PDU linecard 121, in contrast, is dedicated to receipt of PDU traffic. Because TDM fabric 101 is unable to groom the PDU traffic, TDM fabric 101 switches any channels 131 containing PDU traffic to packet fabric 102. All cell switching is performed by packet fabric 102.
Referring to FIG. 2, a prior art configuration of a switching fabric using packet switching is illustrated. In this configuration, PDU linecard 221 transfers ingress PDU traffic to packet fabric 201 to groom the traffic and return it to PDU linecard 221 to be transmitted out to its destination. TDM linecards 211, however, are supported by packet fabric 201 through the use of circuit emulation. Circuit emulators 212 are used at the input and output of packet fabric 201 and from/to TDM linecards 211. Circuit emulators 212 prepare the TDM traffic to be switched over data fabric 201, such as by preparing the TDM traffic to be seen by packet fabric 201 as highest-priority PDU traffic.
Although there are many possible advantages in cost, size, and complexity to provide both TDM and PDU traffic support in a single fabric, the current methods in the art either do not adequately provide for switching of PDU traffic, or they introduce jitter, latency, and/or cell loss in TDM traffic.